1. Field of the Invention
The present invention relates to a speed control apparatus of a combined cycle power generation plant.
2. Description of the Related Art
As a power generation system, a combined cycle power generation plant is known. The control of the combined cycle power generation plant is achieved by a gas turbine control system. A control logic circuit is incorporated in the gas turbine control system to output a gas turbine load instruction. Such a control logic circuit is shown in FIG. 1. Referring to FIG. 1, the control logic is composed of a load limit control circuit 101, a speed governor control circuit 102, a temperature limit control circuit 103 and a fuel limit control circuit 104. The load limit control circuit 101 outputs a load limit control signal (LDCSO) 105. The speed governor control circuit 102 outputs a speed governor control signal (GVCSO) 106. The temperature limit control circuit 103 outputs a temperature limit control signal (TCSO) 107. The fuel limit control circuit 104 outputs a fuel limit control signal (FLCSO) 108. Of the signal lines in the figure, the broken line shows a digital signal and the solid line shows an analog signal.
A minimum level selector 109 is supplied with the load limit control signal 105, the speed governor control signal 106, the temperature limit control signal 107, and the fuel limit control signal 108. The minimum level selector 109 selects a signal with a minimum level L less than  from among the above-mentioned four control signals 105, 106, 107 and 108 and outputs it as a final fuel control output signal (CSO control signal) 110. The final fuel control output signal 110 is a control signal to control a fuel quantity supplied to the gas turbine 111.
A house load operation signal 112 and an over-speed protection control (OPC) operation signal 113 which is sent from an over-speed protection control circuit are supplied to the load limit control circuit 101. The house load operation signal 112 and the over-speed protection control signal 113 are supplied to a logical summation (OR) unit 114. A one-shot timer 115 outputs a signal 116 for a predetermined time period in response to the signal outputted from the logical summation unit 114. The control output signal (CSO) 117 is further supplied to the load limit control circuit 101. The control output signal 117 is supplied to a function value unit 118 and an adder 119. The function value unit 118 converts the control output signal 117 into a value signal. The adder 119 adds the control output signal 117 and the value signal from the function value unit 118. A signal obtained by the addition in the adder 119 is supplied to a rate-added switching unit 120 in which a rate is switched at the same time as a switching operation, in addition to a constant time signal 116. The switching unit 120 outputs not an addition value obtained by the addition in the adder 119 but a signal with a value set by a constant value unit 121 as the load limit control signal 105 in response to the signal 116 for the predetermined time period.
The house load operation signal 112xe2x80x2, a variable value signal (SPSET) 122 which indicates the difference of a load set value and an actual load value, and an axis rotation frequency signal 123 are supplied to the speed governor control circuit 102. The house load operation signal 112xe2x80x2 and the variable value signal 122 are supplied to a proportional integrator 124. The proportional integrator 124 integrates the variable value signal 122 proportionally to produce an integration value output signal 126. Also, the proportional integrator 124 outputs a constant value signal of 0 which is set to a constant value unit 125 in response to the house load operation signal 112xe2x80x2. A subtractor 127 subtracts the axis rotation frequency signal 123 from the integration value output signal 126 to produce a subtraction Resultant signal 128. The subtraction resultant signal 128 is amplified by an amplifier 129 and is outputted as the above-mentioned speed governor control signal 106.
FIG. 2 shows the control logic of the over-speed protection control (OPC) circuit. The rotation frequency signal 131 of the gas turbine 111, an entrance pressure signal 132 in a middle-pressure turbine of the gas turbine 111, a generator current signal 133, and a generator output signal 134 are supplied to function value units 137, 138, 139, and 140 to convert the physical quantities of them into percentage values, respectively. A generator breaker-on signal 135 is supplied to an inverter (NOT) unit 130. A house load operation switching signal 136 is supplied to a one-shot timer 152. The middle-pressure turbine entrance pressure signal 132 and the generator current signal 133 are supplied to a subtractor 142 through the function value units 138 and 139. The subtraction value obtained by the subtractor 142 or, a load difference signal 143 is supplied to an adder 145 through the other function value unit 144. The rotation frequency signal 131 is supplied to the adder 145 through the function value unit 137. The addition value 146 obtained by the adder 145 is supplied to a monitor unit 147 which sends a digital signal based on an optional setting range. The generator output signal 134 is supplied to a logical product (AND) unit 149 through a function value unit 140, a monitor 141, and an off-delay timer 148. The generator breaker-on signal 135 is supplied to the logical product unit 149 through the inverter unit 130. A monitor signal 150-1 outputted from the monitor unit 147, a signal 150-2 outputted from the logical product unit 149 and a signal 150-3 outputted from the one-shot timer 152 are supplied to a logical summation (OR) unit 151. The output signal outputted from the logical summation unit 151 is the above-mentioned over-speed protection control signal (OPC signal) 113 shown in FIG. 1 for the OPC operation.
The over-speed protection control (OPC) circuit has the control logic shown in FIG. 2, and is a protection unit of the gas turbine 111 from over-speed trip which occurs when the turbine 111 is accelerated rapidly in case of rapid load decrease due to load blocking-off and so on. The over-speed protection control (OPC) circuit monitors the speed increase rate as the output signal of the function value unit 137 and the load difference as subtraction value 143. When the value obtained by adding a bias (preceding) signal based on the load difference to the turbine rotation speed increase rate is larger than a threshold value set in the monitor unit 147, the over-speed control (OPC) operation is carried out to close a turbine governor rapidly.
In the above conventional apparatus, there is a case that the OPC operation is not carried out because the switching to a system isolated operation is carried out but a system load loss is small. In such a case, the final fuel control output signal (CSO control signal) 110 is limited based on the load limit control signal 105 which follows a change rate which is determined from the viewpoint of the gas turbine protection or the control signals such as the temperature limit control (TCSO) signal 107. Also, the time period appears during which the axis rotation frequency control cannot be carried out based on the speed governor control (GVCSO) signal 106. At that time, the phenomenon occurs where the rotation frequency decreases largely from a rating range of the rotation frequency, and the trip is caused. Also, when the variable value signal 122 is determined from the difference of a load set value and an actual load value after the switching to the system isolated operation, the variable value signal 122 and the integration value signal 126 change based on an erroneous load set value regardless that the load is not clear. Therefore, there is fear that the integration value signal 126 cannot keep a constant value and influences as an external disturbance. For this reason, the rotation frequency control based on the speed governor control signal 106 as the speed governor control circuit output becomes difficult. Moreover, even when the operation is switched to the system isolated operation and the OPC operation is carried out because the system load loss is large, the rotation frequency increases for this time period and the rotation axis inertia becomes large. In this case, the trip is caused in the process of decreasing the rotation frequency, if there is a large time difference between the switching to the system isolated operation and the OPC operation.
Therefore, it is demanded that an improper control by which the trip phenomenon is caused can be avoided, and a proper control can be carried out on the switching to the system isolated operation.
In conjunction with the above description, an operation control apparatus is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 9-324656). In this reference, a minimum level selector 16a selects a minimum one of the output signal of a fuel limit control unit 11, the output signal of a load limit control unit 12, and an output signal of a speed control 13 and outputs the selected signal to a minimum level selector 16b and an air control valve 4. The output signal of a blade path temperature limit control unit 14 and the output signal of an exhaust gas temperature limit control unit 15 are supplied to the minimum Level selector 16b. The minimum level selector 16b selects a minimum one of the output signal of a blade path temperature limit control unit 14, the output signal of an exhaust gas temperature limit control unit 15, and the output signal of the minimum level selector 16a and outputs the selected signal to a fuel control valve 3.
Therefore, an object of the present invention is to provide a combined cycle power generation plant in which a proper speed control is carried out to prevent a trip.
Another object of the present invention is to provide a combined cycle power generation plant in which proper speed control is carried out on the switching to a system isolated operation.
In an aspect of the present invention, a combined cycle power generation plant includes a gas turbine operating in response to a turbine control signal, and a speed control apparatus which outputs the turbine control signal to the gas turbine based on a speed governor control signal for a first time period after an operation of the plant is switched to a system isolated operation. The speed governor control signal is a signal for speed control of the gas turbine, and a switching signal is generated when the operation of the plant is switched to the system isolated operation.
The speed control apparatus may include a speed governor control circuit, a plurality of limit control circuits and a minimum signal selector. The speed governor control circuit generates the speed governor control signal. The plurality of limit control circuits generate control signals for limit controls to the gas turbine, respectively. The minimum signal selector selects the speed governor control signal from among the speed governor control signal and the control signals for the first time period as a signal with the lowest level.
In this case, it is desirable that at least one of the plurality of limit control circuits outputs the control signal in response to the switching signal to have a predetermined level higher than the lowest level for a second predetermined time period including the first time period.
In this case, the limit control circuit includes a load limit control circuit for maximum output limit control of the gas turbine. The load limit control circuit may include a one-shot timer, a constant value generating unit and a switching unit. The one-shot timer generates a first timer signal for the second predetermined time period in response to the switching signal. The constant value generating unit generates a first constant value signal. The switching unit outputs the first constant value signal in response to the first timer signal as the control signal.
The limit control circuit may include a temperature limit control circuit for maximum temperature limit control of the gas turbine. In this case, the temperature limit control circuit may include a one-shot timer, a constant value generating unit and a switching unit. The one-shot timer generate a second timer signal for the second predetermined time period in response to the switching signal. The constant value generating unit generates a second constant value signal. The switching unit outputs the second constant value signal in response to the second timer signal as the control signal.
Also, the limit control circuit may include a fuel limit control circuit for maximum fuel limit control of the gas turbine. In this case, the temperature limit control circuit may include a one-shot timer, a third constant value generating unit and a switching unit. The one-shot timer generate a third timer signal for the second predetermined time period in response to the switching signal. The third constant value generating unit generates a third constant value signal. The switching unit outputs the third constant value signal in response to the third timer signal as the control signal.
Also, the speed governor control circuit may include a proportional integrator, a subtractor and an amplifier. The proportional integrator proportionally integrates a variable value signal to produce an integration value signal, and the variable value signal indicates a difference between a target load value and an actual load value. The subtractor subtracts an actual rotation frequency indicating the actual rotation frequency of the gas turbine from the integration value signal to produce a subtraction resultant signal. The amplifier amplifies the subtraction resultant signal and outputs the amplified signal as the speed governor control signal.
Also, the speed governor control signal is the signal for the speed control of the gas turbine in a rating speed range, and the speed governor control signal is lowered in level in response to the switching signal. In this case, the speed governor control circuit may include a switching unit, a proportional integrator, a subtractor and an amplifier. The switching unit outputs a variable value signal as a selected signal usually and outputs a predetermined level signal as the selected signal in response to the switching signal, the variable value signal indicating a difference between a target load value and an actual load value. The proportional integrator proportionally integrates the selected signal to produce an integration value signal. The subtractor subtracts an actual rotation frequency indicating the actual rotation frequency of the gas turbine from the integration value signal to produce a subtraction resultant signal. The amplifier amplifies the subtraction resultant signal and outputs the amplified signal as the speed governor control signal.
Also, the speed control apparatus may further include an over-speed control circuit which generates an over-speed control operation signal when a difference between a value signal corresponding to an entrance pressure signal of a middle-pressure turbine of the gas turbine and a value signal corresponding to a generator current is equal to or larger than a predetermined threshold, and when the operation of the plant is not switched to a house load operation. A load limit control circuit may include a one-shot timer, a constant value generating unit and a switching unit. The one-shot timer generate a fourth timer signal for a second predetermined time period in response to the over-speed control operation signal, the second predetermined time period including the first time period. The constant value generating unit generates a fourth constant value signal. The switching unit outputs the fourth constant value signal in response to the fourth timer signal as the control signal.
In another aspect of the present invention, a combined cycle power generation plant includes a gas turbine, a load limit control circuit, a speed governor control circuit and a minimum level selector. The load limit control circuit outputs a load limit control signal to control speed of the gas turbine. The speed governor control circuit outputs a speed governor control signal to control the speed of the gas turbine. The minimum level selector selects the speed governor control signal from among the load limit control signal and the speed governor control signal for a time period after switching to a system isolated operation of the plant. The peed governor control signal is lower than the load limit control signal in level, and a system isolated operation signal is generated when an operation of the plant is switched to the system isolated operation.
In this case, the load limit control circuit may include a first switching unit which outputs the load limit control signal with a level higher than the speed governor control signal in response to the system isolated operation signal.
Also, the speed governor control signal may be a signal with a level corresponding to a subtraction value obtained by subtracting a rotation frequency of the gas turbine from an integration value of a variable value signal with respect to a reference level, and the variable value signal indicates a difference between a load set value and an actual load value. The speed governor control circuit may include a second switching unit which outputs a value with a level lower than the variable value signal in response to the system isolated operation signal in place of the variable value signal.
Also, the load limit control circuit may include a first switching unit which outputs the load limit control signal with a level higher than the speed governor control signal in response to the system isolated operation signal. The speed governor control signal may be a signal with a level corresponding to a subtraction value obtained by subtracting a rotation frequency of the gas turbine from an integration value of a variable value signal with respect to a reference level, and the variable value signal indicates a difference between a load set value and an actual load value. The speed governor control circuit may include a second switching unit which outputs a value with a level lower than the variable value signal in response to the system isolated operation signal in place of the variable value signal.
Also, the combined cycle power generation plant may further include a temperature limit control circuit may include a third switching unit which outputs a temperature limit control signal with a level higher than the speed governor control signal in response to the system isolated operation signal for the predetermined time period.
Also, the combined cycle power generation plant may further include a fuel limit control circuit may include a fourth switching unit which outputs a fuel limit control signal with a level higher than the speed governor control signal in response to the system isolated operation signal for the predetermined time period.
In addition, the combined cycle power generation plant may further include an over-speed control logic circuit which outputs a load loss signal as a logical product of a load difference signal and a logical inversion of the system isolated operation signal.